Same body, same glaze, same firing. Why did one crawl?

Notes

The body: M370. Glaze: G2934Y (with added green stain). Firing: Cone 6 drop-and-hold. Glazing method: dipping (using tongs). Thickness: The same. The difference: Wall thickness. The one on the right was cast thinner so the glaze took a lot longer to dry (the bisque lacked sufficient absorbency). Common pottery glazes contain clays which need to shrink somewhat during drying. The bond with the bisque, although fragile, is normally enough to prevent cracking during drying if: drying occurs quickly. That happens when the body has enough porosity to absorb all the water quickly. Otherwise, cracks appear and these become crawls during firing. A complicating factor is that stain and/or zircon additions make an already-crawl-susceptible glaze even worse. Solution: Heat bisque before dipping, glaze the inside and outside separately (with drying between) or increase calcined kaolin:raw kaolin ratio in the glaze.

Links

Glossary

CrawlingA ceramic glaze fault that occurs during firing of the ware, islands of glaze form as it crawls, leaving bare patches of body.

Troubles

Glaze CrawlingAsk yourself the right questions to figure out the real cause of a glaze crawling issue. Deal with the problem, not the symptoms.